The invention relates generally to the formation of concrete pipe, and particularly presents a solution to the problem of residual strain created by the troweling of zero slump concrete with rotary motion. In the usual pipe forming apparatus, the outer form of the pipe is stationary. Thus, rotary motion by the packerhead imparts a stress to the concrete mix, such stress being particularly disturbing in the event the concrete pipe is supplied with reinforcing wire. It has been found that such wire may be strained beyond its yield point causing permanent bending as well as deflection of the wire, making the completed pipe product unsatisfactory. Furthermore, because of the difference in modulus of elasticity between steel and concrete relieving such undesirably induced stress after the pipe is formed causes unnecessary voids to be formed in the concrete pipe. Obviously, such voids reduce the strength of cured pipe and allow leakage in the pipe through the voids.
In the case of non-reinforced pipe residual stress movement of the concrete causes cracks to form in the pipe thereby reducing strength of the pipe and permitting leaks. As a result of these encountered problems, many purchasers of concrete pipe refuse to accept pipe formed by a rotating packerhead or roller-head method.
The prior art is not entirely devoid of vibrating cores used in the manufacture of concrete pipe. For example, the U.S. Pat. to E. Robbins, No. 1,504,834, illustrates a core for a pipe form inserted into and withdrawn from the form therebeneath by means of controlled fluid pressure. U.S. Pat. No. 1,961,981 to Pechstadt also discloses a movable core for a pipe form, and a tamper for compacting the cementitious material. A further improvement to the basic concept of these two patents is disclosed by U.S. Pat. No. 2,544,453 to Gaudin which illustrates a vertically movable vibrating core insertable into and withdrawable from a pipe form.
However, none of these patents nor any material presently available in the art of forming concrete pipes discloses a solution to the residual stress problem discussed above which is encountered in the current practice of the manufacture of pipe which employs the use of a vertically disposed form and a packerhead which moves slowly along the axis of the form while rotating and while concrete is poured in to form the pipe.
The instant invention overcomes the disadvantages of the prior art by permitting the formation of concrete pipe by the rotating packerhead method without inducing residual stress either in the concrete or in the reinforcing wire, if used. By employing a vibrating core together with a rotating packerhead as the concrete pipe is formed, residual stresses are avoided due to vibratory motion being imparted to the concrete and reinforcing wire during the pipe forming operation. Of further significance is the fact that the vibrating core of this invention may be used on concrete pipe formed by a variety of other methods such as by spinning, vibration, or tamping, before the pipe is cured so as to relieve all residual stress, fill voids, compact and densify the concrete and induce a more intimate bond between reinforcing wire and the concrete mix prior to curing of the pipe. Finally, desirably larger aggregate may be used in the concrete mix for concrete pipes resulting in a decrease in absorption of the pipe wall because vibration by the core will cause thorough mixing of the concrete as well as a better coating of sand and aggregate particles with cement.